1. Field of the Invention
The present invention relates to the production of a mono or polyphase, for example three-phase, AC current and more particularly to the conversion of mechanical energy, obtained for example by virtue of a wind turbine, into electrical energy.
2. Brief Description of the Related Art
It is known to use a cage-type asynchronous machine as alternator, the cage-type asynchronous machine presenting advantages of simplicity and robustness. However, its operation at almost fixed speed may be to within slippage a source of noise due to the recurring modification of the orientation of the blades, and this may engender variations of mechanical torque on the shaft. Such torque variations may result in disturbance to the network, since the current drawn may undergo significant variations. Furthermore, the maximum of the theoretical power cannot be utilized for high wind speeds.
In the case where use is made of a cage-type asynchronous machine connected to the network via a power electronics interface, variable-speed operation is possible by virtue of a variable speed drive. The presence of a rectifier makes it possible to deliver a DC voltage and an inverter makes it possible to deliver an AC voltage of fixed frequency corresponding to that of the network, with a unit power factor. The nominal power of the alternator then determines the maximum power that can be provided by the wind turbine. However, the converters used are rated for the whole of the power exchanged between the alternator and the network. They therefore represent a significant cost, have non-negligible losses, and may give rise to disturbances which are detrimental to the efficiency and quality of the energy delivered. The use of such converters, which are necessarily over-rated for the whole of the nominal power of the generator, may give rise for example to up to 3% of the nominal power as losses.
The rectifier used may be a rectifier controlled under pulse width modulation (PWM). In this case, the transfer of reactive power may be controllable, but the transfer of active power remains the same as when a simple rectifier comprising a diode bridge is used.
In the case where a doubly fed asynchronous machine is used, the stator is linked directly to the network while the power electronics interface is inserted between the rotor and the network thus allowing a span of variation of the slippage and therefore of the rotation speed of the order of 30%. However, this is a machine that is of more complex construction and therefore more expensive and less reliable than the cage-type asynchronous machine. The stator of the machine being wired up directly to the network, it may also undergo large variations in current during network disturbances.
It is also known to use synchronous machines, especially comprising permanent magnets. In particular, the use of a permanent magnet alternator followed by a power electronics interface that can comprise a rectifier followed by an inverter is known for the conversion of wind energy. The span of variation of the rotation speed is typically from 30% to 120% of the nominal rotation speed. However, such alternators exhibit the drawback of significant cost, especially because of the presence of the permanent magnets as well as the impossibility of being able to de-excite the machine in the case of a defect, for example in the case of a short circuit or overspeed.
It is also known to use synchronous machines excited by a winding at the rotor drawing from the network via a power electronics interface that can comprise a rectifier followed by an inverter. The span of variation is of the same order of magnitude as for the magnet-type machine.
Application EP 1 187 307 A2 discloses an electric machine comprising a claw pole rotor, feeding to a converter. A claw pole rotor is characteristic of an electric machine of small size and low power.
U.S. Pat. No. 5,083,039 A discloses an asynchronous cage machine and the regulation acts on the parameters id and iq so as to control the torque or the voltage.
U.S. Pat. No. 6,239,996 B1 relates to a machine intended to charge automobile batteries.
U.S. Pat. No. 6,437,996 B1 is aimed at switching to constant DC voltage so as to decrease the reactive power in order to transport energy with the least losses over several kilometers.
A need therefore exists to employ a wind energy conversion assembly having reduced bulk and reduced manufacturing cost.
A need also exists to benefit from an assembly making it possible to optimize the efficiency obtained.